I had not had my mechanical mod very long and had a battery discharge too quickly because the power switch got stuck in the on position when in a pants pocket; I was not wearing the pants at the time this incident happened.

The battery blew out its ends and melted, and the hot gases were vented out of the battery and the mod as they were designed to do to prevent an explosion. It was still a scarey experience.
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The mod was a BB, and it has a bulbous, protuding power switch as seen in the above photo. You can see how the power switch could get compressed in a tight space. The next photo is of the battery that vented.
This incident was the result of user error. It certainly taught me a lesson on proper battery use and precautions. Practice battery safety at all times, and respect the power these things have in them, and you should be fine.
All of the variable voltage battery devices have built-in safety features which will cause them to shut down before a dangerous situation can occur. Mechanical mods have no such safety features, so stacking batteries in them increases the risk of problems occuring.